THE SOUTHERN MARGIN OF THE ROCKALL PLATEAU - STRATIGRAPHY, TERTIARY VOLCANISM AND PLATE TECTONIC EVOLUTION

The geological evolution of the southwest Rockall Plateau in the area of Edoras Bank has been clarified using seismic reflection, gravity an d magnetic data. Four principal reflectors are observed within the Ter tiary sedimentary sequence: I, Late Miocene: II, latest Early Miocene; III, Late Eocene; IV, earliest Eocene. A period of pronounced sedimen t drift accumulation marks the interval I-II. Reflector III, the top o f a sediment wedge prograding southward from Edoras Bank, marks a chan ge from terrigenous to pelagic sedimentation that is probably related to subsidence of the Rockall Plateau following the separation of Green land from Eurasia in the earliest Eocene. Reflector IV marks the top o f a wedge-shaped seismically transparent layer that also thins southwa rd away from Edoras Bank. On the basis of its seismic attributes and m agnetic signature, this layer is interpreted as a volcanic sheet, form ed as part of the North Atlantic Tertiary Volcanic Province during rif ting of Greenland from Eurasia. The recognition of voluminous volcanic rocks south of Edoras Bank extends the known area of the Tertiary vol canic province several hundred kilometres to the south. Gravity anomal y modelling and continental reconstructions suggest that the region so uth of Edoras Bank is underlain by thinned continental crust. A four s tage geological evolution for this region is indicated. (i) Initial ri fting associated with the separation of Labrador from Greenland in the late Cretaceous is characterized by enhanced crustal thinning and sub sidence in the region of a rift triple junction. (ii) Passive subsiden ce and accumulation of late Cretaceous and earliest Tertiary sediments followed the initiation of seafloor spreading in the Labrador Sea. (i ii) Blanketing of the area by Palaeocene volcanic rocks masked pre-exi sting magnetic lineations, providing an explanation for some of the pr oblems in earlier interpretations based mainly on magnetic data. (iv) Post-volcanic sedimentation, continuing to the present day.